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The Defenders #20
Saga of the Swamp Thing #20 is the first issue written by Alan Moore. Naturally, you'd figure that would be the pick here, right?
Well, while it certainly is an important part of comic book history, that issue isn't really all that well known, is it? Heck, it is not even reprinted in the collected Swamp Thing trades, right? They all start with #21, the much more famous issue.
So, with that in mind, I am going to choose the first issue of a DIFFERENT comic book writer, Steve Gerber's first issue on The Defenders.
Gerber's run on the title was not even all that long, yet during that period, the late, great Gerber put together what you really could argue is one big graphic novel, as he took the Defenders from the differing approaches of Roy Thomas and Steve Englehart and truly melded the team into the "non-team" that we all know and love so well (yes, the "non-team" approach had been already established, more or less, before this, but not to the extent Gerber took it).
It was a great achievement in comic book superhero storytelling, and I think it is fair to celebrate his run with this issue in the countdown (and I DO think that a lot of folks recall Gerber's first issue of Defenders).
Other great #20s include a pair of first appearances in Amazing Spider-Man #20 (Scorpion) and Fantastic Four #20 (Molecule Man).
Sandman #20 and Doom Patrol #20 are also notable issues.